| Reactive gas-solid two-phase flows are widespread in energy conversion,chemical and oil industries, electric power and other fields. A typical industrialapplication is fluidized bed reactors. Fluidized bed reactor has achieved a wideindustry uses over the past decades due to its advantages such as mixingefficiently of particles, ability to control temperature easily, fuel flexibility.In-depth understanding and grasp of the mechanisms of gas-solid flows has verypractical significance. With the advancement of computational fluid dynamics(CFD), numerical simulations have been widely utilized in the research ofgas-solid flows. However, challenges such as gas-solid turbulence, dissipation ofparticle collision and multi-coupling between gas and solid phases increase thecomplexity of numerical simulation. Besides, the coexistence of homogeneousand heterogeneous reactions and variation of temperature due to heat exchange ofreactions lead to severe coupling between the interdependent process of turbulentgas-solid flow and reactive process. Up to now, there is no universal numericalmodel applicable to the reactive gas-solid process in fluidized bed reactor in theopen literature. Current mathematical models from different researchers based onsome assumptions are only valid under certain circumstances. Therefore,development of mathematical models plays an important role in researchingreactive gas-solid flow process.Based on the Large Eddy Simulation (LES) of gas turbulence, the turbulentkinetic energy equations considering interaction of gas-solid phases are present,and the subgrid turbulent energy model is proposed. The transport equations forthe second-order moment (SOM) of particle fluctuating velocity are used,considering anisotropic characteristic of particle velocity fluctuating. For theinteractions between gas and solid phases, not only consider gas-solidinter-phase force, but the second order fluctuating energy are implementedtransferring between gas subgrid turbulent energy and second-order moment ofsolids. The LES-SOM model is presented considering gas turbulence, anisotropyof solid fluctuating velocity and first order and second order interaction betweengas and solid phases. With the extension of LES-SOM model into reactions, theequations of species and energy of gas and solid phases are presented. Applyingthe eddy dissipation concept (EDC) reaction model into LES simulation of gasreactions, by comparing subgrid scale and “fine structure” scale, the subgrid reaction model is presented within fine structure.Numerical simulations of hydrodynamics of gas-solid two-phase flows inrisers are performed based on LES-SOM model. The simulations are in goodagreement with low mass flux experiment results by Jiradilok et al. and highmass flux experiment results by Herbert et al. Simulations predict a typicalcore-annular flow structure of low solid concentration and upward flow in thecore, and high solid concentration and downward flow in the annular region. Thedistribution of subgrid turbulent kinetic energy and subgrid energy dissipation ishigh in the middle and low near the wall. The anisotropic behavior of fluctuatingvelocity of particles is obvious. The axial second-order moment of particle ishigher than radial second-order moment of particles. Dispersed particlesdemonstrate more anisotropic behavior of fluctuating velocity than particles inclusters. The distribution of solid Reynolds stress has the similar trend with thoseof gas phase, but the value is lower. Compare gas-solid flow behaviors withdifferent sub-grid LES model of gas phase and with different gas-solidinteraction model between phases. The effects of different restitution coefficients,superficial gas velocities and mass flow rates are analyzed. As the gas velocityincreases and particle circulating flow reduces, the ratio of axial and radialsecond-order moments is increased as a power function. The ratio can be up to4-4.5. The distribution of particle concentration along radial direction changesfrom "U" type to "inverted U" type, and the distribution is occurred reversal. Itobtained the critical gas superficial velocity and critical particle circulation flowwith diameter and density of particles of300m and2500kg/m~3.The LES-SOM model is applied to simulate the flow and combustionbehavior of coal particles in fluidized bed combustor. The comprehensive coalcombustion model is presented. By comparing simulating results with subgridreaction model of gas phase, the simulating gas components in the outlet agreewell with experiment of Topal et al. Simulations also take coal devolatisation,carbon particles combustion, volatile matter combustion and desulfurizationprocess into account. The results capture the distribution of concentration,velocity, second-order moment, and Reynolds stresses of coal particles anddesulfurizer particles. The distributions of gas components and temperaturefields are studied. The reaction rates of coal combustion process are alsoconsidered as a function of temperature and concentration. The subgrid reactionrates increase with temperature rising. With the increase of particle concentration,volatile homogeneous reaction rates increase, and carbon heterogeneous reaction rates decrease gradually.Considering solids frictional stress in high volume fraction of particles, theLES-SOM model is applied to simulate the gasification process of wood particlesin bubbling fluidized bed. The comprehensive wood gasification model ispresented. By comparing simulating results with subgrid reaction model of gasphase, gas species in the outlet are in agreement with experimental data withinacceptable error. The influence of bubbles on flow and reaction behavior isstudied. It is especially addressed that distribution of particle concentration,velocity, second-order moment and Reynolds stresses of two groups of particleswith different sizes in the gasification process.The process of tar devolatisation,wood gas combustion, char combustion and gasification are also analyzed.Simulation also presents the distributions of gas species molar fraction andtemperature field in the reactor. The reaction rates of biomass gasificationprocess are also considered the relation with temperature and concentration. Thesubgrid reaction rates increase with temperature rising. With the increase ofparticle concentration, homogeneous reaction and carbon oxidation rates increasefirst, reach the maximum, and then decrease gradually. At low particleconcentration, carbon reduction reaction rate increases with the increase ofconcentration, but at high particle concentration, little changes with theconcentration of particles. Water gas reaction and methanation reaction rates areundepedent upon particle concentrations. |
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